Within various electrical generation devices, and particularly within variable frequency (VF) electrical generators and generator control unit (GCU) systems, high voltage can occur when the stationary exciter field current is commanded to a maximum level. Such a situation may occur when, for example, the point-of-regulation (POR) sensing fails, and the GCU commands full field current. The maximum voltage amplitude appearing at the terminals of a synchronous generator during full field excitation is a function of the saturation characteristics of the generator, the magnitude of the stationary field current, and the speed of rotation or electrical generation frequency.
Modern generators operating in, for example, the 400 Hertz (Hz) frequency range are designed to limit the maximum voltage which appears at the terminals of the generator by carefully selecting the operating point on the B-H curve of iron used in the generator. With such careful selection of the B-H operating point, if a full field current event occurs, the machine saturates to produce a maximum output voltage of approximately 150-160 V line-to-neutral (1-n). This maximum value is well below the typical maximum specification value of 180 V 1-n. This type of peak voltage limiting, however, is a function of the physical characteristics, construction and materials, of the generator itself and the speed or frequency at which it is operated. This type of voltage limit may be simply explained as the generator has a maximum available magnetic flux. The voltage output is proportional to this flux and to operating speed.
Many generators are required to operate over widely varying rotational input speeds, for example, as high as 2:1 maximum-to-minimum operating speed. In such cases, since the maximum voltage would change with rotation speed or frequency of the generator, at two times speed the maximum voltage may be 300-320 V 1-n or more. This requires the load equipment be designed to tolerate these higher voltages, which is likely to add to the cost of the equipment and potentially effect the reliability of the equipment.
More traditional approaches have been to provide a "voltage clamp" at the output of the generator that is activated when the voltage exceeds a value, say 170 V 1-n. Alternatively, a very fast responding exciter current controller may be used. Both of these approaches suffer from fault tolerance. Neither system is required to perform until a voltage is too high. An undetected passive failure disabling the exciter current peak voltage controller or the voltage clamp may result in a high voltage condition at the terminals of the generator. This is a very undesirable condition.
It has been known in the art to use compensating coils in generators to perform functions other than peak voltage regulation. For example, U.S. Pat. No. 4,871,960 describes an exciter having two excitation coils each with an associated voltage regulator. The primary coil and associated circuit regulates the generator voltage with a current inversely proportional to speed. The other coil and associated circuit compensates for load and speed fluctuations.
In U.S. Pat. No. 4,885,526 a scheme is described to compensate for the effect of armature reactance on the flux distribution within the machine. This concept supposedly improves the efficiency of the machine if the compensating circuit uses mostly reactive power.
In U.S. Pat. No. 4,176,292 an auxiliary field winding is utilized to provide trim flux along with steady-state flux generated by the main field to accommodate load changes in a superconductive generator.
Thus, a need remains for an improved apparatus and method for peak voltage limiting in generator applications that is both cost-effective and reliable.